1. Field of the Invention
The invention relates generally to the field of automatic gain control of an amplifier, and more specifically to provision of automatic gain control controlled by a plurality of sources.
2. Description of the Prior Art
In a digital data processing system, data is typically stored on magnetic disks in the form of magnetic transitions. Each disk is divided into a plurality of concentric tracks, with each track being at a predetermined location from the center of rotation of the disk. Each disk is also divided into a plurality of sectors, which represent a selected angular region of the disk. In addition, since each disk typically has two sides on which data may be written, the location of data is fully identified by a surface, track and sector identification. If a disk system has more than one disk, a disk identification may also be necessary.
A sector has several portions. A sector begins with an erased portion, followed by a preamble, the data and ends with a postamble. The erased section permits location of the preamble, which is essentially data which has a predetermined initial pattern which is selected to permit rapid timing synchronization. In addition, the preamble includes an address portion to permit identification of the sector, track, surface and disk. Following the address portion is the data.
A disk system typically includes an amplifier for amplifying the read signal from a read/write head. The amplifier's gain, or degree of amplification, is controlled by an automatic gain control (AGC) circuit, which effectively receives the output signal from the amplifier and controls the amplifier to ensure that the output signal is within a selected range. This prevents the output signal from becoming so large as to overload downstream circuitry, which can result in distortion and corruption of the data by the downstream circuitry.
Two problems arise in connection with a disk system in which amplification is controlled by an AGC circuit. The first is that, in trying to locate the erased portion of a sector, the gain level controlled by the AGC circuit cannot be too high or otherwise noise in the erased portion will prevent it from being recognized as the erased portion; the noise will appear erroneously as a recorded signal. The correct choice of gain may also vary over each disk as a result of variations over the disk surface. In addition, the correct choice of gain may vary from disk to disk, as a result of manufacturing variations among disks and heads. However, the correct choice of gain will assist in the speedy location of an erased portion, allowing it to be smaller, and, accordingly, allowing a disk to be divided into more sectors, allowing a disk to hold more data.
The second problem is that, in the data portion of the sector, the data pattern is unpredictable. Thus, there may be relatively long times in which a low signal is generated by the read/write head, as a result of which the AGC circuit will increase the gain of the amplifier. This may also result in the amplification of noise as the data signal, resulting in erroneous data. In addition, when valid transitions are finally detected on the disk, the signal generated by the amplifier may be so large as to cause distorted or corrupted data to be generated and transmitted to downstream circuitry.